we want to send messages (Publish/Subscribe) whenever Camunda Engine has changed something. As we do not want to spam to this topic, we had the idea to publish only when Camunda has completed a transaction to send a message like this:

you can use an execution listener. It is triggered when a process instance / execution is changed. If you need more (e.g. variable changes) then you can implement a custom history event handler. If you need to be aware of the transaction then you can collect the events and register a TransactionSynchronisationListener which pushes the events on commit.